The annual updating of the thesaurus with a new edition enables a) replacement of terms with more up-to-date terminology, b) replacement of ambiguous terms and c) correction of errors. However, these changes may impact the ability to search previously indexed database records for those who use the thesaurus as a controlled vocabulary. The Thesaurus Staff makes all effort to replace deleted or modified preferred terms with a cross-reference to the new preferred term. There are 82 replaced preferred terms between the 2017 and 2018 editions.